Engineering interfacial modification on nanocrystalline hematite photoanodes: A close look into the efficiency parameters
Introduction
The increasing demand of energy of the modern society and the concern on limitation of the fossil fuels sources has encouraged the research on renewable energy sources. Among several alternatives, the hydrogen (H2) production has gathered great attention being considered one of the most promising clean and sustainable future energy sources [[1], [2], [3]].
One with the most elegant and clean solution for H2 production includes the process involving photocatalytic devices composed basically by water, a photosensitive semiconductor responsible for converting photons in usable chemical energy (H2, O2) and sunlight [2,[4], [5], [6]]. Among several semiconductors used as photoanode, hematite is specially promising due to its good chemical stability in aqueous systems, a very favorable optical gap and abundance on the Earth surface. Despite recent innovations, hematite poor optoelectronic properties involving charge separation/transport processes in solid-solid/solid-liquid interfaces as well as the efficiency on photon/energy conversion are still a barrier for limiting the performance of the material [6,7]. The overall efficiency that controls the process remains in coupled parameters defined as ηcat, which considers the chemical reactions on the surfaces and ηsep, which concerns the bulk separation of the generated charges induced by sunlight [8]. Enhanced electronic properties arisen from modification containing for instance Sn4+ [9,10] and Ti4+ [11,12] foreign elements have shown to be a promising way to address hematite electronic limitation. Lately, dual modifications on hematite photoanodes for instance with metal/nonmetal based materials [13] Cobalt based [14] and nickel based compounds deposited over semiconductor surfaces have been reported to act on passivation of intermediate states and enabling hole collection at lower potentials [[15], [16], [17]].
Although the combination of these elements can be promising, the perspective from literature still shows limitation in fully understand how these foreign elements act in hematite photoanodes. This work presents a strategy to effectively decouple and control the efficiency parameters by a meticulous addition of foreign materials into hematite photoanode. For that, the adopted synthesis method allowed the construction of planar and porous nanocrystalline structures with fine thickness control and good reproducibility [18,19]. In contrast to nanorods based structures, planar polycrystalline assemblies allow a higher density of solid/solid interfaces. Such characteristic is desirable once this work concerns on maximization of the modification effect by addition of foreign elements, key parameters for engineering of interfaces.
Section snippets
Design of the photoanode
Ultrathin, bare and modified hematite photoanodes were synthesized via spin coating deposition of iron based polymeric precursor following a recent report [18,19]. In a typical synthesis, iron based inorganic precursor (iron nitrate: Fe(NO3)3.9H2O, Alfa Aesar, 99.5%), citric acid (C6H8O7, J.T.Baker, 99.5%), and ethylene glycol (HOCH2CH2OH, Sigma Aldrich, 99.8%) are dispersed in water. The solution is warmed up to ~70 °C per 30 min to promote chelation of the Iron ions by citric acid followed by
Non-modified hematite photoanodes
Fig. 1 shows the Scanning Electron Microscopy (SEM) micrographs of the fractured non-modified hematite photoanodes via polymeric precursor method. Cross-sectional SEM images reveal a polycrystalline system with nanometric thickness, ranging from ~25 to 130 nm. Examples are shown for photoanodes H-02 and H-05 in Fig. 1a and b, respectively. As can be seen in Fig. 1, the average thickness increment of all synthesized photoanodes is given by the piling of the grains along the microstructure. Such
Conclusion
In summary, the present work concerns in a strategy to engineer hematite electrodes and isolate modification effects at the photoanodes interfaces. For that, a porous and planar morphology using one polymeric precursors and spin coating procedures enabled us to atomically modify the interfaces and surface for catalysis. The selected modifiers, Sn4+ and Ni/Fe, were carefully characterized allowing the elucidation of the role of Sn4+ as responsible for enhanced electronic transport and creation
Declaration of competing interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgment
The authors gratefully acknowledge support from FAPESP (Grants 2017/11986-5 and 2019/01470-7) and Shell and the strategic importance of the support given by ANP (National Agency of Petroleum, Natural Gas, and Biofuels. - Brazil) through the R&D levy regulation. We also thank Otavio Berenguel from Brazilian Nanotechnology National Laboratory (LNNano) for the X-ray diffraction analysis.
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